The present invention is related to an image sensor packaging technique based on a Ball Grid Array (BGA) IC packaging technique, further referred to as image sensor ball grid array (ISBGA).
Digital imaging is becoming a fast growing market, as digital sensors are being used in a variety of applications ranging from professional and personal photography, medical imaging, Internet, to video conferencing and even cellular mobile telephone telecommunication. As is the case for all consumer electronics also the digital imaging market is characterized by an ongoing reduction of costs combined with an increase in performance.
CMOS image sensors are expected to replace Charge Coupled Devices (CCD""s) in the near future as the image sensor technology, particularly in low-cost image capture applications. Using CMOS based image sensor technology allows the integration of the image sensing part with the digital control and signal processing circuitry. This integration can even further reduce the cost of the overall digital imaging and processing circuitry leading to so-called optonic, i.e. opto-electronic devices. Low cost, standard packages are generally available, but these packages are not suitable as they lack a window through which the light or radiation can be sensed. Standard packages may also be too small to contain image sensors, as such sensors including the electronic circuitry can become very large. Custom made packages do allow for window covers, but they are typically too expensive. In view of this development, the packaging of such an image sensor has become an important issue, mainly due to the large contribution of the packaging cost in the overall cost of the image sensor fabrication.
In the article xe2x80x9cActive Pixel Sensor (APS) packagexe2x80x9d, published in the 1999 International conference on High Density Packaging and MCMs, an active pixel sensor package is disclosed. The singulated dies are placed in an array of, for example, ceramic land-grid-array (LGA) packages. The streets between the dies are filled with B-stage epoxy. On each die, a glass frame is attached using this epoxy. Afterwards the array of LGA-packages, containing the glass-covered chips, is sawed into single packages. This package however suffers from adhesion problems on MCM-boards. Also the array-wise packaged dies have characteristics different from dies packaged in an individual way.
In xe2x80x9cA CSP optoelectronic Package for Imaging and Light detection applicationsxe2x80x9d, published by ShellCase, POB 48328, Jerusalem, Israel, an opto-electronic package is presented. A glass plate is attached to the front of the wafer using an optically clear epoxy adhesive. The back of the wafer is ground and etched to separate the dies. A second glass plate is attached to the back of the wafer using an intermediate epoxy layer. This epoxy layer also has to planarise the overall surfaces as, for example, the trenches in the back of the wafer which separate the dies. The bonding pads on the front side of the wafer are connected using a back-side metallization process. The full stack of glass plate, wafer, and backside metallization has then to be diced. Although a hermetic sealing of the device is obtained, the process requires a considerable amount of additional processing steps, such as grinding the backside of the wafer.
Aim of the Invention
An aim of the present invention is to fabricate an Image Sensor Ball Grid Array (ISGBA) package having the light sensitive part of the imager hermetically sealed from the environment. Ball grid array (BGA) packaging allows for packaging of highly functional devices with a large number of I/O""s (inputs/outputs). This is combined with a true die size packaging, leading to a minimum footprint of the package. BGA packages can be produced in a more standard way. These packages are adapted in order to transmit the light or radiation to the sensor and to provide a hermetic sealing of the obtained cavity to prevent e.g. diffusion of moisture or epoxy in between the image sensor and the transparent cover. The image sensor can be connected to the BGA substrate using wire bonding or flip chip bonding.
Another aim of the invention is to fabricate an Image Sensor Ball Grid Array (ISGBA) having the light sensitive part of the imager non-hermetically sealed from the environment. These Non-hermetically sealed ISBGA""s are further referred to as NHISBGA""s. This non-hermetic sealing offers reliability at least similar to standard plastic packaged components, but is produced in a more cost-effective way compared to the ISGBA. The distance between the image sensor and the transparent cover is controlled, thereby avoiding e.g. the occurrence of Newton-ring effects between the transparent cover and the die.
The present invention may provide a Hermetically Sealed Ball Grid Array device, comprising a structure having a sealed cavity, wherein said cavity is defined by walls according to a closed geometric configuration between a first substrate and a second substrate;
said second substrate being transparent;
said first substrate being a semiconductor substrate containing at least one optically sensitive area;
said walls comprising a metal seal between the first and second substrate and said walls surrounding said at least one optically sensitive area. The metal seal may comprise a stack of layers including at least a first metallization layer, a reflowed solder layer, and a second metallization layer. The second substrate may be larger than said first substrate, in which case the solder balls may be attached to said second substrate.
The present invention may also provide a Sealed Ball Grid Array device comprising,
a first substrate, said first substrate being a semiconductor substrate containing at least one optically sensitive area;
a second transparent substrate sealingly attached to and covering said optically sensitive area of said first substrate;
said device further comprising a third substrate, said third substrate being attached to said device; and
said first substrate being electrically connected to said third substrate using wire bonding, said wire bonds being totally encapsulated by a polymer. The device may further comprise a cavity, said cavity being defined by walls between said first substrate and said second substrate, said walls being located around said at least one optically sensitive area. The walls may comprise a stack of layers comprising at least a polymeric dam or a metal seal. The dam may comprise a structure to control the distance between said first and said second substrate or a separate spacer may be provided to control the distance between said first and said second substrate. The second substrate may be sealingly attached to said first substrate using a glue layer, said glue layer and said second substrate covering said at least one optical sensitive area formed on said first substrate. Preferably, the glue layer is a photo-sensitive material, for example BCB.
The devices described above may be used in a CMOS imager device
The present invention also includes a method for fabricating a Hermetically Sealed Ball Grid Array device, comprising the steps of
providing a first substrate;
said first substrate being a semiconductor substrate containing at least one optically sensitive area;
defining a pattern of solderable material for contact pads and sealing ring on said first substrate;
providing a second transparent substrate;
defining a pattern of solderable material for forming a sealing ring on said second transparent substrate; and
assembling said second substrate to said first substrate by means of said sealing ring pattern. The method may further comprise the steps of:
dicing said second transparent substrate into individual transparent covers after defining the pattern of solderable material thereon;
using the individual covers in the assembly step;
dicing said first substrate into individual dies and
packaging said individual dies into a Ball Grid Array package.
Alternatively, the method may further comprise the steps of:
dicing said first substrate after forming the pattern of solderable material thereon;
the step of defining the solderable material on said second substrate includes defining an interconnect pattern thereon;
the assembly step includes assembling said diced first substrate to said second substrate by means of said sealing ring pattern using a flip chip assembly technique;
underfilling of the gap between said diced first substrate, said second transparent substrate and said sealing ring; and
attaching solder balls to said interconnect pattern on said second substrate; and
dicing said second transparent substrate.
The attachment of the solder balls may further comprise the steps of
dicing the second substrate;
attaching an interposer frame on said second substrate, said interposer frame containing a window in which said diced first substrate fits, and
attaching the solder balls to said inter-poser frame and dicing of said interposer frame.
The present invention may provide a method for fabricating a Sealed Ball Grid Array device, comprising the steps of
providing a first substrate;
said first substrate being a semiconductor substrate containing at least one optically sensitive area;
attaching said first substrate to a third substrate;
forming wire bonds between the bond pads of said first substrate to said BGA package;
depositing a dam on said first substrate at the inside of said wire bonds;
depositing a polymer over said wire bonds to completely encapsulate said the wire bonds, said dam preventing the flow of said polymer over said first substrate;
providing a second transparent substrate and
attaching said second substrate to said polymer on said first substrate curing said polymer.
The present invention may also provide a method for fabricating a Sealed Ball Grid Array device, comprising the steps of
providing a first substrate;
said first substrate being a semiconductor substrate containing at least one optically sensitive area;
coating a thin optical glue layer on said first substrate;
patterning said glue layer;
providing a second transparent substrate;
coating a thin optical glue layer on said second substrate;
patterning said glue layer;
attaching said second substrate to said first substrate;
dicing said first substrate; and
packaging said diced first substrate into a Ball Grid Array package, said packaging comprising the steps of
forming wire bonds between the bond pads of said diced first substrate to said Ball Grid Array package and
depositing polymer over said wire bonds to completely encapsulate said the wire bonds.
The present invention will be described with reference to the following drawings.